Ethernet networks are frame-based computer networks for local area networks. In order to more efficiently use the available bandwidth in a switched Ethernet network with multicast data streams the so called IGMP (Internet Group Management Protocol) snooping mechanism exists.
The Internet Group Management Protocol (IGMP) is a communication protocol used to manage the membership of Internet Protocol multicast groups. Therefore, IGMP is used by IP hosts and adjacent multicast routers to establish multicast memberships. IGMP snooping is the process of listening to IGMP traffic and allows the switch to listen in IGMP conversation between hosts and routers by processing the layer 3 IGMP packets sent in a multicast network.
In a switched Ethernet network without IGMP snooping all multicast traffic is broadcasted to any port of the Ethernet switch. This means that there is no difference between a real broadcast and multicast traffic at the switched network. Therefore, the multicast behaviour is only present at the routed level of the network. By use of IGMP snooping the switch will only send the multicast stream to switch-ports that have shown interest in the multicast stream. At other switch-ports no bandwidth is used for the multicast stream. However, an IGMP querier is required for this mechanism to function. Usually, the IGMP querier is implemented in the Ethernet router. But this could also be the (root)switch. In a network comprising many multicast streams IGMP snooping is inevitable.
There are solutions for a switched Ethernet network to create a redundant network path. This can be handled by standardized protocols like Spanning Tree Protocol (STP IEEE 802.1d) and Rapid Spanning Tree Protocol (RSTP IEEE 802.1w). Since an Ethernet network does not allow for loops these protocols find the loop in the network and interrupt it automatically. In case that the communication path is interrupted somewhere else (e.g. cable failure) this failure is detected and the place where the loop was broken on purpose is repaired.
If IGMP snooping is combined with (R)STP a problem occurs when one of the communications is interrupted. That is because the multicast stream is not present at the alternative link and therefore, it can not be distributed downwards the network. At regular intervals the IGMP querier sends out a multicast message to all hosts in the network if they do (still) require a multicast stream. When this message arrives at the host, via the backup path, the host will respond. From that moment onwards the host will receive the multicast stream.
As the IGMP querier interval is 125 s in Commercial Of The Shelve (COTS) equipment the host can be without the multicast stream for maximal 125 s. In many applications this is far to long